The present invention relates to a direction finder with an array antenna, for measuring the incoming angles of plural incident signals to the array antenna in multipath environments such as urban spaces or indoors. The present invention also relates to a device for processing measurement results for the direction finder.
The MUSIC method or ESPRIT method is known as an algorithm for measuring the incoming angles of plural radio signal with no correlation that are received by an array antenna. These algorithms, which can provide excellent resolution and precision, compared with, for example, the conventional Fourier conversion method or the beam scanning method, have been widely applied to direction finders. However, the algorithm cannot accurately perform a specific value resolution because the factorial of a covariance matrix of a signal received by an array antenna is degenerated when correlative signals such as multipath waves are handled. This means that the incoming angle cannot be measured.
An approach of overcoming the above-mentioned drawback, is the movement averaging method In this method a matrix created by dividing a covariance matrix into plural matrix groups with equal orders, as shown in FIG. 3, is decomposed into specific values, and the factors of the divided matrix groups added together. Such a method is disclosed in the literature, "On Spatial Smoothing for Direction of Arrival Estimation of Coherent Signals", IEEE Trans on ASSP Vol. 33(4), 1985. In order to apply the movement averaging method, it is essential that the covariance matrix is in a Vandermode format. In the array antenna for a direction finder, the antenna elements are arranged at regular intervals. The received phase response (a) must be expressed as shown by the following formula (1): EQU a=-exp(jnkd sin .theta.) (1)
where n is the number of an antenna element; k is a wave number; d is an element spacing; and .theta. is an incoming angle.
However, the movement averaging method cannot be applied to an actual array antenna because the received phase response (a) is not linearly expressed as shown by the formula (1) due to the mutual coupling between antenna elements even when antenna elements are arranged at regular intervals. Many literatures disclose the technique of measuring the incoming directions of plural correlative signals using the above-mentioned movement averaging method. In any of the techniques, the incoming angle is measured by spatially moving a sole antenna element at regular intervals to avoid the interference of the mutual coupling. Any trouble does not occur even when an antenna element is spatially moved in a static environment such as indoor environment limiting no measurement time. However, the conventional technique is impractical in the case when the antenna element is mechanically moved in outdoor mobile communication environments where the propagation condition momentarily changes, in consideration of the measurement time.